1. Field of the Invention
The present invention relates to a radial tire for a levitation-type vehicle, which is used for example a the time of landing and taking-off of a linear-motor car which is levitated and travels under the action of magnetism.
2. Description of the Related Art
At present, a magnetic levitation-type vehicle (hereinafter referred to as "linear-motor car") has been developed. The linear-motor car has a vehicle body which is levitated from a road surface of a U-shaped cross-section guide way, by magnetic force acting between the vehicle body and the road surface. The magnetic force acting between sidewalls of the guide way and the vehicle applies propulsive force to the vehicle body to run the vehicle body along the guide way under a non-contact condition. Theoretically, the levitation height may be of the order of a few millimeters. However, in consideration of a characteristic of Japan that it is a country having frequent occurrence of earthquakes, a superconductive magnet is used to create a strong magnetic field in order to achieve a levitation height of the order of 100 mm.
In connection with the above, in such a linear-motor car, it is necessary to support and guide the vehicle body with respect to the road surface at landing and taking-off. To this end, tires for the linear-motor car have been developed.
As shown in FIG. 5, the load, which is received by the tires of the linear-motor car, changes or varies depending upon time and speed. That is, at the beginning of taking-off, the tires receive all of the load of the linear-motor car, but since a magnetic levitation force increases in accordance with an increase in the speed, the load applied to the tires decreases gradually, and comes to zero after taking-off. Further, at landing, the load is applied gradually to the tires from a non-load condition accompanied by a decrease in the speed, in contrast to the aforesaid taking-off condition. After stopping, the tires receive all of the load of the linear-motor car. In this manner, the condition at which the load applied to the tires varies depending upon time, does not exist in the case of tires which are used for an ordinary automotive vehicle. Further, the tires for the linear-motor car are also different from those for aircraft, and are under a slip condition between the tires and the surface of the roadway at landing for a relatively long period of time within a low load range. Accordingly, a wear condition of the tires is completely different from that of the tires for aircraft. With the tires for the linear-motor car, which are used under special conditions at landing and taking-off, the tires are in contact with the road surface particularly at landing under the low load for a relatively long period of time. Accordingly, in the case where conventional tires are used for the linear-motor car, a ground-contact configuration of the linear-motor car tire at the time of full load (at a load of 100%) has, as shown in FIG. 4a, a ground-contact length A of a central portion longer than a ground-contact length B of each of a pair of shoulders. Thus, within the low-load range, i.e., at a load equal to less than 45% of the full load of the vehicle, only the central portion of the tire in the widthwise direction thereof is in contact with the ground as shown in FIG. 4b. Consequently, wear on the tread (crown) of the tire, particularly, on the central portion of the tire in the widthwise direction, i.e., slip wear on the central portion increases. Accordingly, there is an urgent need for development of tires exclusively for use on the linear-motor car.
In other words, at landing, the tires of the linear-motor car are in sliding contact with the road surface at high speed for a relatively long period of time under a low-load condition. Accordingly, there is a particular increase in wear on the central portion of the crown of the tire so it is necessary to reduce the wear in this region.
Referring to FIGS. 4c and 4d, a mechanism of wear on the widthwise central portion of the tire of a conventional tire applied to a linear-motor car having a form in which the widthwise central portion projects will be decreased.
In FIG. 4c, a point e indicates the widthwise central portion of the tire, a point h indicates a contact point with the road surface in the neighborhood of one of the shoulders of the tire, and points f and g indicate intermediate points between them. Further, in FIG. 4c, in the condition indicated by the solid lines only the point e is in contact with the road surface, while in the condition indicated by the double dotted lines the crown of the tire is in contact with the road surface over its entirety in the widthwise direction thereof.
In FIG. 4d, the area indicated by E represents the amount of wear at point e. The area indicated by F represents the amount of wear at point f. The area indicated by G represents the amount of wear at point g. The area indicated by H represents the amount of wear at point h. Further, in FIG. 4d, the amount of wear (W) can be expressed by the following relationship: EQU W.varies. (Vehicle Speed).sup.2 .times.(Ground-Contact Pressure At Each Point Of Crown On The Basis Of Vehicle Load)
In connection with the above equation, the vehicle speed was a strong correlation to dependence on the slip ratio.
On the basis of FIG. 4d, the total amounts of wear at the respective various points are related as follows: EQU E&gt;F&gt;G&gt;H
As a result, it will be understood that the amount of wear at point e, that is, at the widthwise central portion of the tire is extremely large.
On the basis of FIG. 4d, the reason why the amount of wear of the tire is particularly large at the point of time when the load applied to the tire is equal to or less than 45% of the full load is that a slip phenomenon is particularly large between the tire and the road surface up until this point of time. If the load applied to the tire exceeds 45% of the full load, the slip phenomenon becomes very much less.
Further, FIG. 4e shows a comparison between an amount of wear of the conventional tire (small in radius of curvature (CR)), an amount of wear of a tire where the radius of curvature of the crown of the tire is large, and where the radius of curvature of the crown of the tire is extremely large with respect to time. In FIG. 4e, the upper graph shows a comparison between the conventional tire and a tire where the radius of curvature of the crown is large. The lower graph shows a comparison between the conventional tire and a tire where the radius of curvature of the crown of the tire is extremely large, that is, where the crown has a nearly concave configuration. Moreover, in FIG. 4e, the points e, f, g and h indicate points the same as those shown in FIG. 4c. The solid lines represent the conventional tire. The broken lines indicate the tire where the radius of curvature of the crown is large, while the chain lines reveal the case of a tire where the radius of curvature of the crown is extremely large. FIG. 4e shows the amount of wear from landing start to stop at the points e, f, g and h. The areas covered by the respective curves represent the amounts of wear at the respective points. From FIG. 4e, the tire in the case where the radius of curvature of the crown is extremely large has a larger amount of wear at the ground-contact point (point h) in the vicinity of the shoulder of the tire than the widthwise center (point e) of the tire. That is, it can be seen that the area covered by the curve for point h&gt;the area covered by the curve for point e. In this connection, the total amount of wear of the tire is substantially the total sum of the areas covered by the respective curves which represent the respective points, and the total amount of wear of the aforesaid three types of tires is substantially the same.